fix(HAI-060): remove nested scrollbar from activity in card detail view

- Remove overflow CSS rules causing nested scrollbar in activity section
- Add TaskDetailModal test asserting activity list scroll behavior
- Clean up merger test formatting

packages/dashboard/app/components/__tests__/TaskDetailModal.test.tsx

@@ -214,4 +214,29 @@ describe("TaskDetailModal", () => {
     );
     expect(withoutTitle.querySelector(".detail-id")?.textContent).toBe("HAI-099");
   });
+
+  it("activity list does not have nested scroll constraints", () => {
+    const { container } = render(
+      <TaskDetailModal
+        task={makeTask({
+          log: [
+            { timestamp: "2026-01-01T00:00:00Z", message: "Created task" },
+            { timestamp: "2026-01-01T00:01:00Z", message: "Started work" },
+            { timestamp: "2026-01-01T00:02:00Z", message: "Completed step 1" },
+          ],
+        })}
+        onClose={noop}
+        onMoveTask={noopMove}
+        onDeleteTask={noopDelete}
+        onMergeTask={noopMerge}
+        addToast={noop}
+      />,
+    );
+
+    const activityList = container.querySelector(".detail-activity-list");
+    expect(activityList).toBeTruthy();
+    const style = (activityList as HTMLElement).style;
+    expect(style.overflowY).not.toBe("auto");
+    expect(style.maxHeight).toBe("");
+  });
 });

packages/dashboard/app/styles.css

@@ -663,17 +663,11 @@ html, body {
 }
 
 .detail-activity-list {
-  max-height: 260px;
-  overflow-y: auto;
   display: flex;
   flex-direction: column;
   gap: 0;
 }
 
-.detail-activity-list::-webkit-scrollbar { width: 4px; }
-.detail-activity-list::-webkit-scrollbar-track { background: transparent; }
-.detail-activity-list::-webkit-scrollbar-thumb { background: var(--border); border-radius: 4px; }
-
 .detail-log-entry {
   padding: 8px 0 8px 14px;
   border-left: 2px solid var(--border);